remove Vercel AI Gateway and Vercel Sandbox (#33067)

* remove Vercel AI Gateway provider and Vercel Sandbox terminal backend

Both Vercel-hosted integrations are removed end-to-end. Users on the AI
Gateway should switch to OpenRouter or one of the other aggregators
(Nous Portal, Kilo Code). Users on the Vercel Sandbox backend should
switch to Docker, Modal, Daytona, or SSH.

What's removed:
- `plugins/model-providers/ai-gateway/` provider plugin
- `hermes_cli/vercel_auth.py` Vercel-Sandbox auth helper
- `tools/environments/vercel_sandbox.py` terminal backend
- `ai-gateway` provider wiring across auth, doctor, setup, models,
  config, status, providers, main, web_server, model_normalize, dump
- `vercel_sandbox` backend wiring across terminal_tool, file_tools,
  code_execution_tool, file_operations, approval, skills_tool,
  environments/local, credential_files, lazy_deps, prompt_builder,
  cli, gateway/run
- `AI_GATEWAY_BASE_URL` constant, `_AI_GATEWAY_HEADERS` auxiliary-client
  header set, run_agent base-URL header/reasoning special-cases
- `[vercel]` pyproject extra and `vercel`/`vercel-workers` from uv.lock
- env vars: `AI_GATEWAY_API_KEY`, `AI_GATEWAY_BASE_URL`, `VERCEL_TOKEN`,
  `VERCEL_PROJECT_ID`, `VERCEL_TEAM_ID`, `VERCEL_OIDC_TOKEN`,
  `TERMINAL_VERCEL_RUNTIME`
- Tests: deletes test_ai_gateway_models.py and
  test_vercel_sandbox_environment.py; scrubs references across 23
  surviving test files (no entire tests deleted unless they were
  dedicated to AI Gateway / Sandbox)
- Docs: provider tables, env-var reference, setup guides, security
  notes, tool config, terminal-backend tables — English plus zh-Hans
  i18n parity
- `hermes-agent` skill: provider table entry and remote-backend list

What stays (intentional):
- `popular-web-designs/templates/vercel.md` — CSS design reference,
  unrelated to Vercel-the-AI-product
- `x-vercel-id` in `stream_diag.py` headers — generic Vercel CDN
  response header, useful diag signal on any Vercel-hosted endpoint
- `vercel-labs/agent-browser` URL in browser config — lightpanda
  browser project, different OSS effort
- `userStories.json` historical contributor entry mentioning Vercel
  Sandbox — archive, not active docs

Validation:
- 1153 tests in the 22 targeted files pass (`scripts/run_tests.sh`)
- Full repo `py_compile` clean
- Live import of every touched module + invariant check (no
  `ai-gateway` in `PROVIDER_REGISTRY`, no `_AI_GATEWAY_HEADERS`, no
  `vercel_sandbox` in `_REMOTE_TERMINAL_BACKENDS`)

* test: convert profile-count check from change-detector to invariant

The hardcoded "== 34" assertion broke when ai-gateway was removed.
Per AGENTS.md change-detector-test guidance, assert the relationship
(registry count >= number of plugin dirs) instead of a literal count.
Counts shift when providers are added/removed; that's expected.

tests/providers/test_plugin_discovery.py

@@ -46,14 +46,26 @@ def test_bundled_plugins_discovered():
         assert (child / "plugin.yaml").exists(), f"{child.name} missing plugin.yaml"
 
 
-def test_all_34_profiles_register():
-    """After discovery, the registry must contain exactly 34 distinct profiles."""
+def test_all_profiles_register():
+    """After discovery, the registry must contain every bundled provider directory.
+
+    This is an invariant — the number of profiles matches the number of plugin
+    directories, not a hardcoded count. Counts shift when providers are
+    added/removed; that's expected and shouldn't break CI.
+    """
     _clear_provider_caches()
     from providers import list_providers
 
+    plugins_dir = REPO_ROOT / "plugins" / "model-providers"
+    plugin_dir_count = sum(1 for c in plugins_dir.iterdir() if c.is_dir())
+
     profiles = list_providers()
     names = sorted(p.name for p in profiles)
-    assert len(names) == 34, f"Expected 34 profiles, got {len(names)}: {names}"
+    # Some plugin __init__.py files register multiple profiles, so the registry
+    # count is >= the directory count (never less).
+    assert len(names) >= plugin_dir_count, (
+        f"Expected at least {plugin_dir_count} profiles (one per plugin dir), got {len(names)}: {names}"
+    )
 
     # Spot-check representative providers from different categories
     for required in (